Entropy coding

Entropy coding

An encoder for encoding a sequence of symbols is described which includes an assigner configured to assign a number of parameters to each symbol of the sequence of symbols based on information contained within previous symbols of the sequence of symbols; a plurality of entropy encoders each of which is configured to convert the symbols forwarded to the respective entropy encoder into a respective bitstream; and a selector configured to forward each symbol to a selected one of the plurality of entropy encoders, the selection depending on the number of parameters assigned to the respective symbol.Related Terms:EntropyEncoderEncodingEntropy CodingSelector

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending International Application No. PCT/EP2011/055528, filed Apr. 8, 2011, which is incorporated herein by reference in its entirety, and additionally claims priority from International Application No. PCT/EP2010/054828, filed Apr. 13, 2010, European Application EP 10159793.8, filed Apr. 13, 2010, and U.S. Application 61/432,875, filed Jan. 14, 2011, which are all incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to entropy coding and may be used in applications such as, for example, video and audio compression.

The present invention describes a new method and apparatus for entropy encoding and decoding of discrete data. Entropy coding, in general, can be considered as the most generic form of lossless data compression. Lossless compression aims to represent discrete data with fewer bits than needed for the original data representation but without any loss of information. Discrete data can be given in the form of text, graphics, images, video, audio, speech, facsimile, medical data, meteorological data, financial data, or any other form of digital data.

In entropy coding, the specific high-level characteristics of the underlying discrete data source are often neglected. Consequently, any data source is considered to be given as a sequence of source symbols that takes values in a given m-ary alphabet and that is characterized by a corresponding (discrete) probability distribution {p1, . . . , pm}. In these abstract settings, the lower bound of any entropy coding method in terms of expected codeword length in bits per symbol is given by the entropy

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Huffman codes and arithmetic codes are well-known examples of practical codes capable of approximating the entropy limit (in a certain sense). For a fixed probability distribution, Huffman codes are relatively easy to construct. The most attractive property of Huffman codes is that its implementation can be efficiently realized by the use of variable-length code (VLC) tables. However, when dealing with time-varying source statistics, i.e., changing symbol probabilities, the adaptation of the Huffman code and its corresponding VLC tables is quite demanding, both in terms of algorithmic complexity as well as in terms of implementation costs. Also, in the case of having a dominant alphabet value with pk>0.5, the redundancy of the corresponding Huffman code (without using any alphabet extension such as run length coding) may be quite substantial. Another shortcoming of Huffman codes is given by the fact that in case of dealing with higher-order probability modeling, multiple sets of VLC tables may be necessitated. Arithmetic coding, on the other hand, while being substantially more complex than VLC, offers the advantage of a more consistent and adequate handling when coping with adaptive and higher-order probability modeling as well as with the case of highly skewed probability distributions. Actually, this characteristic basically results from the fact that arithmetic coding provides a mechanism, at least conceptually, to map any given value of probability estimate in a more or less direct way to a portion of the resulting codeword. Being provided with such an interface, arithmetic coding allows for a clean separation between the tasks of probability modeling and probability estimation, on the one hand, and the actual entropy coding, i.e., mapping of a symbols to codewords, on the other hand.

For the sake of clarity, we will restrict the exposition in the following to the case of a binary alphabet, although conceptually, the very basic methods of this invention also apply to the general case of an m-ary alphabet with m>2. The probability distribution of symbols with values in a binary alphabet can be described by a single parameter p=pLPS, where the useful distinction is made between the so-called less probable symbol (LPS) with probability estimates pLPS≦0.5 and the more probable symbol (MPS) with pMPS=1−pLPS. Thus, in the general case we have 0<pLPS≦0.5, where in practical cases, the probability estimate is often bounded from below by a minimum probability value pmin such that pmin≦pLPS.

SUMMARY

According to an embodiment, an encoder for encoding a sequence of symbols may have: an assigner configured to assign a number of parameters to each symbol of the sequence of symbols based on information contained within previous symbols of the sequence of symbols; a plurality of entropy encoders each of which is configured to convert the symbols forwarded to the respective entropy encoder into a respective bitstream; and a selector configured to forward each symbol to a selected one of the plurality of entropy encoders, the selection depending on the number of parameters assigned to the respective symbol.

According to another embodiment, a method for encoding a sequence of symbols may have the steps of: assigning a number of parameters to each symbol of the sequence of symbols based on information contained within previous symbols of the sequence of symbols; and forwarding each symbol to a selected one of a plurality of entropy encoders, the selection depending on the number of parameters assigned to the respective symbol, and each of the plurality of entropy encoders being configured to convert symbols forwarded to the respective entropy encoder into a respective bitstream.

According to another embodiment, a decoder for reconstructing a sequence of symbols may have: a plurality of entropy decoders, each of which is configured to convert a respective bitstream into symbols; an assigner configured to assign a number of parameters to each symbol of a sequence of symbols to be reconstructed based on information contained within previously reconstructed symbols of the sequence of symbols; and a selector configured to retrieve each symbol of the sequence of symbols to be reconstructed from a selected one of the plurality of entropy decoders, the selection depending on the number of parameters defined to the respective symbol.

Another embodiment may have a device for decoding a sequence of symbols sequentially representing syntax elements of a significance mapping and then symbol representations of values of transform coefficients being unequal to zero for blocks of (video) pictures containing transform coefficients being unequal to zero, wherein the syntax elements of the significance mapping specify for the positions of the transform coefficients in a scan order as to whether at the respective position a transform coefficient being unequal to zero or not, and the symbol representations of values of transform coefficients being unequal to zero are represented by the sequence of symbols in a reverse scan order—starting from the last transform coefficient being unequal to zero, the device being configured to reconstruct the sequence of symbols using an inventive decoder.

According to another embodiment, an apparatus for entropy decoding entropy-encoded information symbols, the entropy-encoded information symbols being produced by entropy encoding a symbol of a sequence of symbols based on probability information for the symbol, the symbol being part of a symbol set, wherein the probability information for the symbol is derived based on a context of the symbol, the context including one or more context symbols processed earlier, and wherein, for entropy encoding the start symbol, an initialization probability information was used, the initialization probability information being based on an estimation of a symbol statistics relating to a start symbol and being determined such that an initialization probability distribution is different from an equi-probable distribution for all symbols of the symbol set may have: an inventive decoder for reconstructing the sequence of symbols, the decoder being configured to obtain the probability information having been used when entropy encoding the sequence of information symbols, the decoder including an initializer for obtaining the initialization probability information having been used when entropy encoding the start symbol.

According to another embodiment, a method for reconstructing a sequence of symbols may have the steps of: assigning a number of parameters to each symbol of a sequence of symbols to be reconstructed based on information contained within previously reconstructed symbols of the sequence of symbols; and retrieving each symbol of the sequence of symbols to be reconstructed from a selected one of a plurality of entropy decoders, the selection depending on the number of parameters defined to the respective symbol, and each of the plurality of entropy decoders being configured to convert a respective bitstream into symbols.

Another embodiment may have a computer readable digital storage medium having stored thereon a computer program having a program code for performing, when running on a computer, one of the inventive methods.

An encoder for encoding a sequence of symbols according to an embodiment of the present invention comprises an assigner configured to assign a number of parameters to each symbol of the sequence of symbols based on information contained within previous symbols of the sequence of symbols; a plurality of entropy encoders each of which is configured to convert the symbols forwarded to the respective entropy encoder into a respective bitstream; and a selector configured to forward each symbol to a selected one of the plurality of entropy encoders, the selection depending on the number of parameters assigned to the respective symbol.

A decoder for reconstructing a sequence of symbols according to an embodiment of the present invention comprises a plurality of entropy decoders, each of which is configured to convert a respective bitstream into symbols; an assigner configured to assign a number of parameters to each symbol of a sequence of symbols to be reconstructed based on information contained within previously reconstructed symbols of the sequence of symbols; and a selector configured to retrieve each symbol of the sequence of symbols to be reconstructed from a selected one of the plurality of entropy decoders, the selection depending on the number of parameters defined to the respective symbol.

Embodiments of an entropy encoding algorithm are described in which a sequence of arbitrary source symbols is mapped onto two or more partial bitstreams, and a decoding algorithm, in which the original source symbols are decoded from the two or more partial bitstreams. At the encoder side, the source symbols are first binarized (if they don\'t already represent binary symbols) and each bin of the binarizations is associated with a set of parameters. The associated parameters are then mapped onto a small set of indices and all bins that are associated with a particular index are coded by a particular binary entropy encoder and the corresponding codeword or codewords are written to a particular partial bitstream. At the decoder side, the source symbols are decoded by requests for source symbols. The same binarization scheme as for the encoder is used to convert these request for source symbols into requests for bins, and each request for a bin is associated with the same set of parameters as the corresponding bin at the encoder side. The associated parameters are again mapped onto a small set of indices, where the mapping is the same as at the encoder side. And all bin requests that are associated with a particular index are decoded by a particular binary entropy decoder, which reads a codeword or multiple codewords from the corresponding partial bitstream. The encoders and decoders are configured in a way that different binary entropy encoders and decoders use different coding algorithms (i.e., a different mapping between bin sequences and codewords).

In an embodiment of the invention, the set of associated parameters represents a measure for the probability of one of the two bin values for a particular bin (or a set of equivalent measures). The measure for the probability can for example represent a probability state, which can be represented by a value of a set of N values. The probability estimation is decoupled from the actual coding of the bins. The probability measure is then mapped onto a probability index, by which the encoding and decoding of a bin is assigned to a particular binary entropy encoder and decoder. The mapping of the probability measures to probability indexes (and thus the clustering into groups of probability measures) can also be varied over time, e.g. in dependence of already transmitted symbols. For instance, the assignment can be adapted to the number of bins that have been coded with a certain probability measure or probability index in order to ensure that the created partial bitstreams have similar bit rates. In an embodiment of the invention, the number of binary entropy encoders and decoders is less than the number of possible values for the probability measure. A large number of possible probability measures, which allows an accurate estimation of the associated probability, is mapped onto a small number of probability indices, where each of those is associated with a particular binary entropy encoder and decoder. The small number of binary entropy encoders and decoders provides the advantage that the overhead for transmitting the partial bitstream can be kept small and that the synchronization overhead is of encoding and decoding system can be kept small. When using a probability measure as basis for assigning the bins to particular binary entropy encoders and decoders, the binary encoders and decoders can be particularly optimized for a representative of the group of probability measures, which allows a high coding efficiency (similar to the best known entropy coding schemes). In addition, simple encoding and decoding algorithms, which may consist of a simple mapping of bin sequences to codewords and vice versa, can be designed for the representatives of the groups of probability measure. This reduces the complexity of the encoding and decoding system, while still providing a high coding efficiency. It this point, it should be noted that a high granularity of probability measure is necessitated for an accurate estimation of the bin probabilities; any inaccuracy would have a negative impact on the coding efficiency. But for coding the bins, a relatively small number (significantly smaller than the number of possible probability measure values for the probability estimation) of probability groups is sufficient, since the rate increase that results from encoding a bin with a particular associated probability measure by using an entropy coding algorithm that was designed for a probability that is different than but close to the particular probability is very small. And using a small number of binary entropy encoders and decoders provides the above mentioned advantages. In further embodiments of the invention, the set of parameters that is associated with a bin consists of a measure for the probability of one of the two bin values and one or more additional parameters.

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